Re:Sexy prime
by enedil on Friday February 15, 2019 @08:15PM
(#19176342)
Using Euclid's algorithm (not Euler's) is certainly not the easiest way of checking for divisibility by 3 - a number is divisible by 3 if and only if it's sum of digits is divisible by 3. You can repeat the process until you have one digit.
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But what if you start with a binary representation?
Do it in quaternary. Add pairs of bits.
Or you can form the alternating sum of the bits, e.g. for 0b10011001 you calculate 1-0+0-1+1-0+0-1 = 0 which is divisible by three. (That's similar to the divisibility test by 11 of a number in base-10, or more generally testing if a number in base `b` is divisible by b+1)